Such highly loadable rolling bearings often have considerable dimensions in the running race diameter of the rolling bearing outer ring. Even a relatively minor distortion of the rolling bearing ring during the heat treatment has a considerable effect, and may necessitate expensive mechanical finishing. Furthermore, the material removal over the circumference may be nonuniform in the event of ovality.
Rings for such applications are preferably made from high-alloy case-hardened steels. Such high-alloy case-hardened steels are marketed for example by Carpenter Technology Corporation/Carpenter Steel Division (USA) under the known brand name Pyrowear 675 Stainless (P675), and are described in the company brochure Stainless Steels 68.2-96-4M and with the following typical composition:    Carbon: 0.07%    Manganese: 0.65%    Silicon: 0.40%    Chromium: 13.00%    Molybdenum: 1.80%    Cobalt: 5.40%    Nickel: 2.60%    Vanadium: 0.60%    Iron: remainder.
Another steel suitable for such uses is available under the designation Carburizing Stainless Steel (CSS) CSS-42L from the Latrobe Steel Company, USA, and according to the company brochure has the following typical composition:    Carbon: 0.15%    Manganese: N/A    Silicon: N/A    Chromium: 14.00%    Molybdenum: 4.75%    Cobalt: 12.50%    Nickel: 2.00%    Vanadium: 0.60%    Iron: remainder.
Another suitable high-alloy case-hardened steel is known under the designation CX13VDW of the manufacturer Aubert & Duval Holding, France, and has the following typical composition:    Carbon: 0.12%    Manganese: N/A    Silicon: N/A    Chromium: 12.00%    Molybdenum: 1.60%    Cobalt: 0    Nickel: 2.50%    Vanadium: 0.30%    Iron: remainder.
U.S. Pat. No. 6,179,933 discloses rolling bearing components carrying rolling bodies—for example for use in aircraft engines—made of high-alloy case-hardened steels (for example the aforementioned Pyrowear 675), which have a specially hardened running surface for the rolling bodies owing to an additional heat treatment of the prehardened steel.
To this end, the rolling bearing component, already prehardened by conventional carburizing known per se and subsequent quenching, is subjected for a short time to a subsequent nitriding treatment (also referred to below as nitriding for brevity). Temperatures in the range of from 375° to 592° C. are proposed, to which the rolling bearing component is exposed in short-term nitriding for from 1 to 2 hours. A very thin, additionally hardened layer of up to 150 μm is thereby formed, with a Vickers hardness of from 800 to 1200 HV 0.3. This layer serves to counteract the customarily increased wear of the running surface during operating startup, or running in.
As mentioned, the high-alloy case-hardened steel must be subjected to a carburizing process known per se in order to generate a surface hardness and layer thickness sufficient for continuous operation. To this end, the case-hardened steel is exposed to carbon at relatively high temperatures of about 850° C. to 1050° C. in an atmosphere containing carbon, and subsequently quenched. Carbon atoms diffuse into the marginal layer of the steel, which itself has only a relatively low carbon content and is therefore not hardenable per se. Below the marginal layer, the steel furthermore desirably remains relatively low in carbon and therefore relatively ductile. The carbon diffusing in is substantially accommodated between the atoms in the lattice of the case-hardened steel. Owing to the quenching, the carbon atoms remain in constrained positions in the atomic lattice of the steel and therefore lead to the hardness increase, while with slower cooling they occupy energetically favorable positions in the lattice without causing a substantial hardness increase.
By pure carburizing, the hardness in the relevant marginal zone (also referred to below as the functional layer) of the rolling bearing component is limited conventionally to less than 1000 HV, the hardness being associated with the formation of relatively large carbides. These reduce the rolling strength. Particularly in the high-tempered state (with a tempering temperature of about 500° C.), the rolling bearing component's corrosion resistance of the surface hardened only by carburizing is low.
Patent Application US 2005/0268990 A1 discloses a heat treatment for case-hardened steel, in particular bearings for aeronautical and astronautical applications, which is likewise based on the previously described carburizing with the problems described above. As US 2005/0268990 A1 describes with the aid of its exemplary embodiments, here again it is possible to use the case-hardened steel Pyrowear 675 which is initially processed mechanically to form the desired bearing component. The bearing component is subsequently carbonitrided. During this carbonitriding process, the bearing component is exposed to a gas mixture having carbon and nitrogen components in a temperature range of from 898° C. to 1093° C. (1650° F. to 2000° F.) for from 40 to 200 hours. The rolling bearing component must subsequently be subjected to quenching, as described above in connection with pure carburizing. This raises the problems explained in the introduction, particularly with respect to dimensional accuracy of the rolling bearing component. During the subsequent mechanical material removal to (re)produce the dimensional accuracy, it is then necessary to remove a relatively thick layer.
Against this background, it is an object of the present invention to provide a method for hardening running surfaces of rolling bearing components, in particular outer bearing rings, with which the rolling bearing component has a layer of particular hardness and corrosion strength in its marginal zone (functional layer).
It is another object of the invention to produce this functional layer in such a way that elaborate subsequent mechanical processing, associated with high material removal, is no longer necessary.